THE GRAIN SIZE CHARACTERISTICS OF LACUSTRINE SEDIMENTS OF XIAZANGTAN LANDSLIDE LAKE AND ITS PALAEOCLIMATIC SIGNIFICANCE IN THE NORTHEASTERN TIBET PLATEAU SINCE THE LAST GLACIAL
There are some controversies on understanding the climate changes since the Last Glacial in the northeastern of Tibetan Plateau (TP),and some traditional views pointed out that climate change in the region responded to the global climate in trend, closing to the Marine oxygen isotope stage (MIS),in other words,the MIS even stages performance for cold,dry climate and the odd stages represent warm and humid environment. However,some scientists hold the opposite opinions. In 2007,the authors surveyed in the Xiazangtan super large scale landslide mass in the Qunke-Jianzha Basin of upper reaches of Yellow River, where confined by Lajishan Mountain and Jishishan Mountain,located at the summer and winter monsoon transition zone, and found a lacustrine-loess sediments profile (35°58'49.52N, 101°59'24.25E; 2316m a.s.l.) in the middle of landslide mass, which depth is about 25.1m, then collected 208 grain size and magnetic susceptibility samples as indicators to test, and used the component separation method of grain size for fractionation. In this paper,the chorology,grain size and magnetic susceptibility of lacustrine and loess sediments profile was studied in detail since the Last Glacial, and mainly achieved the following conclusions: (1) The lacustrine profile of Xiazangtan landslide lake was formed after 727.2ka B.P. and ended about 8.00.9ka B.P., the sediments are mainly composed of silt and clay. (2) The grain size of lacustrine-loess sediments reflected the hydrodynamic conditions,which indicated there are two wet periods 58~50ka B.P. and 30~15ka B.P., and these two periods are also the super large landslides occurred periods. (3) The water level of the Xiazangtan landslide lake are during shallow and depth of cyclical fluctuations since Last Glacial,which reflected the climate change in the northeast margin of the Tibet Plateau represented a dry and wet cyclical change. (4) From the temporal correspondence,the results of this paper may indicate the more humid climatic conditions in the traditional cooler stages in this area.